1. Field of the Invention
The invention relates to the area of metallurgy and relates particularly to an agent for desulphurating steel, comprising high concentrations of SiO2, C2S, and calcium aluminate, and the use thereof in the desulphuration of steel.
2. Description of the Related Art.
Steel manufacture can be carried out schematically in two ways:                transforming iron ore into steel by means such as blast furnaces or converters, and        processing scrap iron in an electric furnace.        
It is known that the presence of impurities, phosphorus and sulphur in the steel obtained after refining cast iron is particularly harmful to mechanical properties. It is a known fact that the presence of a high proportion of sulphur in steel obtained after purification of cast iron produced by blast furnaces is particularly harmful because the sulphur reduces the cold ductility, the impact resistance, and the quality of the ingot surface. The proportion of sulphur that can be tolerated in the metal must be very low, that is to say, under 0.02% or even under 0.005%.
One of the major steps in the current process for steel production is primary metallurgy, by converter or electric furnace, which gives steel that is then reprocessed in a ladle in order to give it specific properties. The most notable progress in the area of improving the properties of steel has been from ladle metallurgy.
Devices for purifying cast iron and producing steel (blast furnaces, converters) make it possible to reduce the sulphur content of the metal, however they do not lead to the total desulphuration that would be necessary to remove the aforementioned disadvantages, whence the need to refine the steel. The general principles of refining can be summarised as described in the text that follows.
In order to extract the impurities from the steel, it has to be put in close contact with a product that has a greater affinity for the impurities, which therefore possesses a lower free enthalpy. This is a problem of thermodynamic equilibrium which can be solved by using high temperatures.
In order to lower the concentration in components that are deemed to have a noxious effect on the steel, the main methods of refining are:    1—exchange through a slag    2—forming insoluble compounds    3—decreasing the solubility of the impurities in the steel by lowering their partial pressure by applying a vacuum to the steel.
The chemical reaction for the desulphuration of steel is as follows:[S]m+(O−)s→(S−)s+[O]m,wherein [S]m and [O]m are the components dissolved in the metal, and (O−)s and (S−)s are the components dissolved in the slag.
A usual method for lowering the concentration in components that are deemed to have a noxious effect on the steel is to use a lime-based slag: In this case, the reaction would be as follows:[S]m+(CaO)s→(CaS2)s+[O]m wherein [S]m and [O]m are the components dissolved in the metal, and (CaO)s and (CaS2)s are the components dissolved in the slag.
As an indication, Table 1 lists in % by weight the usual mineralogical and/or chemical compositions of steelworks slag.
TABLE 1FreeC2SFerriteCaOWustitePericlaseCTiO2V2O5Cr2O3MnOMin1510 1 3 20.020.30.2 0.200.5Max40501520150.3 1.50.520.0010  ZnOCoONiOCuOPbOBaOSrOP2O5SNa2OMin0.010.00010.010.0050.00010.0010.0010.050.010.05Max0.5 0.001 0.5 0.5 0.005 0.5 0.05 2  2.000.5 K2OZrO2MoOBeOTlSn2O3As2O3CdOClFMin0.020.020.00010.00010   0.00010.00010.00010.050.0001Max0.5 0.5 0.001 0.001 0.00050.2  0.005 0.05 2.000.5  
Among the methods currently used for desulphuration however, none is totally satisfying.
Thus the use of sodium carbonate results in a yield of the order of 60% maximum of desulphuration with emission of noxious smoke and the production of particularly aggressive slag.
The use of calcium carbide results in recarburising the metal, and also, the product must be kept dry to avoid the risk of producing acetylene thus causing an explosion.
The use of calcium cyanamide results in nitriding and carburising the metal, which is what is trying to be avoided.
Magnesium is difficult to use because it vaporises on contact with the steel and can result in explosions, and so must be coated in tar and placed in a bell.
The use of silico-calcium, blown into the mass to be purified results in globularisation of the inclusions, and requires the use of alkaline slag and causes the steel to regain nitrogen.
The use of lime is advantageous, but its high melting point, about 2200° C., stops the lime reacting with the liquid metal.
Much research has led to the conclusion that a product with sound desulphuration qualities could contain 53 to 55% of CaO, 43 to 45% of Al2O3 and 1% of FeO. Many products exist with this type of composition such as those described in the French patent FR2541310, filed on 18 Feb. 1983 or the products available from Wacker and also the slag from vanadium production.
However, these products are expensive or not readily available.